1. Field of the Invention
This invention relates to a composting system and more particularly to a composting system which utilizes gravity and natural thermal convection to provide a compact, modular, plug-flow compost reactor which requires minimal aeration and agitation energy.
2. Description of the Related Art
Commercial compost facilities currently employ processes such as mechanically-turned windrows, aerated static piles (covered and uncovered), rotary drum reactors, agitated tunnel configurations, and various in-vessel systems. In some cases, two or more of these processes are combined in a sequence to achieve optimal composting conditions and the highest level of biological stability in the final product.
Mechanically-turned windrow systems employ a mechanical agitation device that is propelled through the composting material usually formed into a windrow of trapezoidal or triangular cross section. The mechanical agitation serves to introduce oxygen into the pile and also increase exposure of material to biological activity through the mechanical disruption of undigested material. Historically, the principal advantages of this system were the low capital equipment cost and minimal site preparation requirements. Time has shown that the disadvantages of this system are 1) long residence or process times to achieve biological stability, 2) lack of distributed temperature and oxygen uniformity through the material, 3) lack of odor control, and 4) negative process impacts from environmental conditions such as daily/seasonal ambient temperature and humidity swings and precipitation events. The lack of odor control has resulted in the closure of more compost facilities than any other cause.
Aerated static pile processes have been employed with the process material (1) directly exposed to the atmosphere, (2) protected from the atmosphere by either impermeable or semi-permeable membranes placed directly over the material, or (3) operated under enclosures. Aerated static pile processes rely on the forced movement of air through the process material to maintain adequate temperature and oxygen conditions while using minimal mechanical agitation of the material. Advantages of this system compared to mechanically-turned windrows include (1) a smaller footprint, (2) a higher level of control over temperature, moisture, and oxygen conditions, (3) somewhat shorter processing times, (4) reduction in the frequency of odor releases and intensity and (5) generally lower labor requirements. Disadvantages include (1) less than optimal exposure of the material surface to biological degradation and (2) pockets of minimally decomposed material due to the lack of sufficient agitation. Uncovered aerated static piles are subject to undesirable odor releases and facilities employing covered piles have had odor problems during periods when unprocessed and processed materials have been uncovered for agitation or during movement of the composting material to the next phase of the process.
The rotary drum systems generally have considerably higher equipment costs than either the agitated windrow or aerated static pile processes due to the need for large reactor vessels and accompanying structural support requirements. Because of the high equipment costs, rotary drum systems are usually designed for minimal material residence times to accommodate only the thermophilic compost stage. Therefore, rotary drum systems usually require deployment of a secondary compost process such as an agitated windrow, agitated tunnel, or aerated static pile to achieve acceptable stability levels.
Agitated tunnel systems use a mechanical agitator to periodically turn and transport the process material along the length of the tunnel throughout the compost process. Forced aeration of the material is usually incorporated into the tunnel floor to maintain active control over temperature and oxygen conditions during the process. The agitated tunnel system is particularly adaptable to large commercial facilities as it minimizes the overall footprint and is very adaptable to a high level of process control in either partially- or fully-automated facilities. It is also well suited to a variety of sites as it can be readily enclosed, if necessary, in harsh climates. Disadvantages of the agitated tunnel system include the amount and precision of the civil work required to support and construct the tunnels and the material handling equipment, particularly in enclosed, automated facilities.
Furthermore, the initial cost and on-going maintenance requirements of the agitator and other material handling equipment often discourage investment in these systems. External energy requirements, either electrical or thermal, can become a significant cost factor, particularly in cold climates.
In-vessel systems define any compost process that uses a special containment unit to create and maintain the ideal environment for the biological degradation of organic materials. In-vessel systems typically monitor and control temperature, oxygen, and sometimes humidity or moisture levels within the containment unit thus creating the conditions for rapid microbial digestion of organic materials. This optimization of process conditions minimizes the processing time required for stabilization of the material. By their very nature, in-vessel systems are typically batch processes and most are not well suited for automation of the material handling requirements. Only a few in-vessel systems incorporate a means of agitation without total removal and replacement of the material at some point during the process cycle. In-vessel systems are more adaptable to small or mid-sized compost facilities.
In many situations, the selection of a compost process is also dictated by the available space; therefore, minimizing the footprint of an installation becomes a critical siting factor. In some cases, a compost facility is only required temporarily at a given site and is subsequently moved; therefore, portability of the equipment is a desirable characteristic.
Accordingly, there is a need for a novel compost system and method that offers a plug flow compost process while incorporating the beneficial aeration, agitation, and process control features found only selectively among other compost systems but in a modular form that requires only minimal footprint, operator attention, external energy and civil work